The DNS SPF Record of webpagetest.org does not allow the server it is using.
Received-SPF: softfail (google.com: domain of transitioning noreply@webpagetest.org does not designate 64.79.149.92 as permitted sender) client-ip=64.79.149.92;
This results on email from noreply@webpagetest.org very often appear as SPAM and are flagged as such.
Cheers!